1. Field of the Invention
The present invention relates to a machine learning device, a CNC device and a machine learning method for detecting an indication of an occurrence of chatter in a tool for a machine tool.
2. Description of the Related Art
Conventionally, in, for example, machine tools controlled by CNC (computer numerical control) devices, chatter sometimes occurs in a specific machining pass owing to the progression of wear of tools, even under the same machining conditions. More specifically, for example, in a machining cycle such as thread cutting, the deeper a thread is cut, the larger the size of the contact area of the tool tip, and the more chatter tends to occur owing to the progression of wear.
An occurrence of chatter (occurrence of chatter in a tool for a machine tool) causes poor machining of workpieces. Especially when a machine tool is brought into an automatic operation during evening hours, if one workpiece has chatter marks, every workpiece machined thereafter will also have chatter marks, thus producing a large number of poor workpieces. Therefore, it is desired to detect the occurrence of chatter in a tool or to detect an indication of the occurrence of chatter in the tool, in order to reduce the number of poor workpieces.
Conventionally, for example, Japanese Laid-Open Patent Publication No. 2012-152835 (Patent Document 1) discloses a vibration determination device having improved chatter vibration detection accuracy by preventing false detection of unstable vibrations caused by vibration of the machine tool itself, an abrupt disturbance vibration and machining, in a machine tool that performs machining while rotating a tool or a workpiece. The vibration determination device determines that the vibration is a chatter vibration when the maximum value of vibration acceleration continues beyond a threshold value for a certain period of time, and the frequency variation in the vibration acceleration is within a certain range.
Conventionally, for example, Japanese Laid-Open Patent Publication No. 2013-036912 (Patent Document 2) discloses a chatter vibration detection device that may prevent poor machining caused by chatter vibration, may select machining conditions in which chatter vibration does not occur, and may reliably detect chatter vibration. In such a chatter vibration detection device, a first vibration is measured at some point in time, and a second vibration is measured after the lapse of a certain time, to calculate first and second power spectra of the first and second vibrations, respectively. The degree of increase in the magnitude of vibration of the second power spectrum, with respect to the magnitude of vibration of the first power spectrum, is calculated. When the degree of increase is equal to or greater than a certain value, it is determined that chatter vibration has occurred.
As described above, conventionally, various proposals have been made to reduce poor machining of workpieces by reliably detecting chatter vibration. However, the actual chatter occurrence mode is different depending on the machining program, the status of use of the tool, and the like.
Thus, for example, in the technique described in the patent document 1, the vibration acceleration (and the continuous time and variation of the vibration acceleration) is used as a determination parameter to prevent false detection, but it is difficult to obtain sufficient detection accuracy unless an appropriate reference value (threshold value) is determined in accordance with the machining properties.
Moreover, for example, in the technique described in Patent Document 2, since an increase in the vibration of an entire frequency band from the vibration of a constant frequency band at some point in time is compared with a reference value (threshold value), there is no need to set a frequency band in accordance with a status of use of a tool. However, it is still difficult to obtain sufficient detection accuracy unless an appropriate reference value (threshold value) is determined in accordance with machining properties.
In other words, the techniques described in Patent Documents 1 and 2 are not possible to detect chatter with high accuracy while flexibly responding to various chatter occurrence modes, which are different depending on the machining program and the status of use of a tool.
Considering the above-described problems of the prior art, an object of the present invention is to reduce the number of poor workpieces by detecting an indication of the occurrence of chatter in a tool for a machining tool. In other words, an object of the present invention is to reduce the number of poor workpieces by easily detecting an indication of the occurrence of chatter in a tool for a machining tool, without examining appropriate reference values nor changing of settings in accordance with machining properties. This application mainly describes the occurrence of chatter during machining, but the occurrence of chatter is often caused by damage to the tool, and as a matter of course, the present invention is effective at detecting damage to the tool.